Most bicycles have multiple gears with front and rear derailleurs to change gears. A front derailleur is used to shift a bicycle chain between two or more chain rings of a crankset. Chain rings vary in diameter, each having a different number of teeth for engagement with the bicycle chain. The force required to turn the crankset is determined, in part, by the size of the particular chain ring the bicycle chain is engaged with. The size of the chain ring can also determine the number of revolutions that will be required of the crankset in order to travel a certain distance. By changing the chain engagement from a chain ring of one size to another, the bicyclist can strategically choose how much force will be required to be applied to the crankset, as well as how many revolutions the crankset will make in order to travel a relative distance.
Conventional bicycle frames are configured with a seat tube joined at one end to a down tube with a bottom bracket lug. The bottom bracket lug supports the “bottom bracket” consisting of the bottom bracket spindle, bearings, races, lock rings, spacers and the like, that are configured to couple with, and allow rotation of, the crankset. The opposite end of the seat tube conventionally supports a seat post and seat for the rider. A front derailleur is typically mounted on the seat tube of a bicycle frame and near the chain rings. Not all bicycle frames have a complete seat tube. For example, some full-suspension mountain bikes have a short seat tube stub for mounting a front derailleur that serves the purpose of a frame structural member for mounting the front derailleur. For simplicity of discussion, it will be assumed that the term “seat tube” denotes a full conventional seat tube or the more recent short seat tube stub for mounting the front derailleur.
A crankset attaches to opposite ends of a bottom bracket spindle. One arm of the crankset is simply a crank arm designed to engage the bottom bracket spindle and a pedal. The other arm of a crankset may include up to 3 chain rings, which require a front derailleur to shift the chain from chain ring to chain ring. The chain rings are typically arranged so as to be concentric with, and parallel to, each other. The smallest chain ring is typically closest to the bicycle frame with the chain rings progressively growing in diameter as they get further from the bike frame. The purpose of a front derailleur is to urge the bicycle chain laterally away from the currently engaged chain ring and towards a chain ring selected by the rider through means of a properly connected shifting device also known as a “shifter.” Front derailleurs of various designs have been utilized to perform this function and most have similar features.
Construction and operation of a basic front derailleur is described in U.S. Pat. No. 4,734,083 to Nagano. The Nagano front derailleur includes a member for fixing the front derailleur to the bicycle, a four pin linkage mechanism to effect a pantographic type movement of two guide members, the guide members typically being designated as the inner guide member and the outer guide member. When the assembly is activated to move away from the bicycle frame and toward a larger chain ring, the inner guide pushes the bicycle chain laterally away from the bike frame until the bicycle chain leaves the currently engaged chain ring and engages the next chain ring. The action of urging the bicycle chain onto a larger chain ring often involves the step of pressing the bicycle chain against the side of the larger chain ring. By pressing the bicycle chain against the chain ring, friction combines with the circular motion of the chain ring to “pull” the bicycle chain up and onto the chain ring. When the assembly is activated to move toward the bicycle frame and toward a smaller chain ring, the outer guide urges the bicycle chain in a similar manner, with the outer guide pressing the bicycle chain off of the larger chain ring, and then allowing the bicycle chain to fall onto the smaller chain ring.
The inner and outer guides are spaced at a width which is wider than that of the bicycle chain, often one and a half to two times as wide as the bicycle chain. This allows the bicycle chain to run between the guides without contacting the guides while also allowing for some lateral movement of the bicycle chain which occurs when the bicycle chain is changed from one gear to another by a rear derailleur on a rear sprocket set also referred to as a “cluster” or “cassette”. The inner and outer guides each typically comprise a substantially flat elongated surface. The guides are placed substantially parallel to one another and are connected together with a front and rear link. An elongated spatial channel, often referred to as a cage is, thus, defined by the inner and outer guides and the front and rear links. The bicycle chain then passes through the cage without touching the inner and outer guides or the front and rear links during normal operation. There are typically multiple links of chain within the spatial area defined by the cage at any given time.
A typical front derailleur requires periodic adjustment to prevent the bicycle chain from engaging an undesired sprocket, or disengaging all sprockets (also known as derailing). A derailed bicycle chain causes complete loss of power transmission to the driving wheel. Bicycle chain engagement with an undesired chain ring causes the bicyclist to either apply more or less force to the crankset, or perform fewer or greater crankset revolutions than would be desired. Such problems are particularly annoying to the bicyclist when ascending a slope. Because the cage of a derailleur is significantly wider than the width of the bicycle chain, a slight misadjustment of the front derailleur can allow for either of the above situations to arise during normal bicycling activities.
One solution for bicycle chain derailment is found in a mechanism referred to as a chain guide. The general design and function of a chain guide can be seen in U.S. Pat. No. 5,782,714 to Osgood. The Osgood device is a stationary member affixed to the bicycle seat tube to prevent the bicycle chain from disengaging laterally toward the bicycle frame from the chain ring closest to the bicycle frame. The Osgood device thus places a limit on the inward movement of the bicycle chain but does not prevent a derailment of the outermost (largest) chain ring.
Another approach to bicycle chain derailment is a second outer chain guide placed on the outside of the largest chain ring. This outer chain guide is circular, typically a slightly larger diameter than that of the largest chain ring, and is placed concentric with, and adjacent to, the largest chain ring. This outer chain guide places a limit on the outward movement of the bicycle chain. However, an outer chain guide such as this is often cumbersome and unsightly. Neither of these chain guides prevent the bicycle chain from disengaging the selected chain ring to engage with a non-selected chain ring.
Thus, there exists a need in the art for a front derailleur with a chain guide that prevents derailments, both inward and outward, and also prevents the bicycle chain from disengaging a selected chain ring and engaging a non-selected chain ring.